What Einstein proposed was that time was a fourth dimension, rather than the fourth dimension. Gravity is also an example of a fourth dimension- all masses cause a "dip" into spacetime in the direction of the fourth dimension, thus drawing other masses towards them.
And the fourth dimension can be worked out simply by extrapolation: 1D- side to side, 2D- side to side, backward and forwards, 3D- side to side, backwards and forwards, up and down, 4D- backwards and forwards, up and down, and an additional direction.